Co-continuous dispersions of incompatible polymers in polymer matrices

ABSTRACT

A method of making and composition for a co-continuous dispersion of incompatible polymers in a polymer matrix comprises forming a solution of the incompatible polymers in a solvent which is common to both polymers and rapidly evaporating the solvent. The remaining material is in the form of the polymer matrix.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of the inventor's previous applicationSer. No. 729,529 filed May 2, 1985, now U.S. Pat. No. 4,594,371 which inturn is a continuation-in-part application of the inventor's previousapplication having Ser. No. 646,311 filed Aug. 31, 1984, now abandoned.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to polymer matrices, and inparticular to a new and useful method of dispersing fine particles ofdifferent incompatible polymers into one polymer matrix.

ABS is an important thermoplastic having U.S. markets of several billionpounds per year with selling prices of $0.50 to $1.00 per pound. It is atwo-phase polymer. The continuous or matrix phase is a copolymer ofstyrene and acrylonitrile typically formed by a free-radicalpolymerization and typically containing 25 to 35% acrylonitrile. Byitself, this matrix phase would be hard and glossy when injection moldedbut would lack impact strength. Major improvements in practicaltoughness are possible when a polybutadiene rubber is incorporated intothe plastic as a dispersed phase.

Several processes exist for incorporation of rubber into a continuouspolymer matrix. Physical blends of rubber and the polymer are possibleusing intensive shear devices such as extruders, but these give aproduct of poor quality. A superior product is formed by the so-calledemulsion process where fine particles of rubber are created byemulsification and hwere the matrix phase is formed by copolymerizationof styrene and acrylonitrile in the presence of these particles. Thisprocess gives an impact-resistant product having good molded gloss. Theaverage rubber particle size is typically less than one micron.

The emulsion process is relatively expensive, and substantial effortshave been devoted to replacing it with a cheaper, bulk process,analogous to bulk processes commonly used for the manufacture of impactpolystyrene. In these bulk processes, a polybutadiene rubber isdissolved in the monomers which will ultimately be polymerized to formthe matrix phase, these being styrene and acrylonitrile in the case ofABS.

The polymerization is initiated by heat or chemical means and takesplace in an agitated vessel. The vessel can be operated as a batchreactor but continuous flow is more common in modern processes. As thepolymer is made, it forms a separate phase from that of the rubber. Thesystem consists of two liquid phases, one being rubber dissolved in themonomeric mixture and the other being newly formed copolymer dissolvedin the monomeric mixture. A phase diagram for the system is illustratedin FIG. 1.

The feed to an agitated vessel consists of rubber and monomers and is inthe single phase region at 10. The effluent stream from the agitatedvessel contains rubber, monomers, and (co) polymers at 11 and is in thetwo phase region 12. Typically, the agitated vessel will be operatedsuch that the polymer-rich phase has a substantially greater volume thanthe rubber-rich phase and will thus be the continuous phase. Therubber-rich phase will be distributed into more or less sphericaldroplets the size of which depends on operating conditions within theagitated vessel and noteably on the speed of the agitator. In practice,rubber particle sizes of 2 to 5 microns are achieved by this process.Particles of 1 micron or smaller are difficult or impossible to achieveeven with very high levels of agitation.

Rubber particle sizes in the range of 2 to 5 microns are suitable formost grades of impact polystyrene but are unsuitable for ABS for whichgood molded gloss in a product requirement. This fact has lead to theabondonment of bulk processes for molding grades of ABS or to thecreation of hybrid processes in which small rubber particles areachieved through emulsification.

SUMMARY OF THE INVENTION

The present invention provides a new means of forming rubber particlesin a polymer matrix. The new technique allows control over rubberparticle size which is independent of the polymerization process andwhich is not critically dependent on hydrodynamic shear forces, inducedthrough agitation.

Accordingly, an object of the present invention is to provide a methodof dispersing one polymer in a matrix of another. This is accomplishedby creating a single phase mixture of the two polymers using a commonsolvent of the two polymers and then rapidly evaporating the solvent toleave the desired dispersion.

A further object of the present invention is to provide a compositionformed of two different incompatible polymers in a single polymer matrixwhich are finely divided into each other.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of the prior art showing how two polymerstypically a rubber and another polymer, and one or more monomers formtwo phases over most compositions, at region 12, during the course ofthe polymerization, the reaction mass gradually evolving from 10 to 11by the relatively slow process of polymerization;

FIG. 2 is a diagram similar to FIG. 1 showing the principle of theinvention wherein a solvent is used; and

FIG. 3 is a detailed diagram similar to FIG. 2 showing an embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the phase diagram of FIG. 2, this diagram is similar toFIG. 1 in that two polymeric components are present, rubber at 25 andthe polymer at 26 which will form the matrix phase of the finishedproduct in area 22. The third component at 24 is a common solvent forthe polymeric components. It can be the monomer(s) of FIG. 1 at 14 or itcan be a non-polymeric solvent such as toluene. The polymer may be acopolymer of styrene and acrylonitrile and the rubber may bepolybutadiene rubber.

According to the invention, a single phase, three component mixture asillustrated in the phase diagram of FIG. 2 is taken as the startingpoint, and then the solvent undergoes rapid evaporation to drive themixture into the two phase region 22. This evaporation can occur in thecomplete absence of polymerization or mechanically induced hydrodynamicforces although such phenomena may occur incidentally without alteringthe essential nature of the process. Advantageously, the solventevaporation will proceed to an extent that the resulting polymericmixture will have such a high viscosity that growth of rubber particlesthrough agglomeration is minimized. Further stabilization of the rubberparticles will occur when the unevaporated solvent is a monomer whichcrosslinks or grafts onto the dispersed particles.

The size of the rubber particles is controlled by two main factors: theratio of rubber to matrix polymer in the initial mixture and the rate ofevaporation. Low rubber concentrations and high evaporation rates favorsmall rubber particle sizes. Surface active agents such asstyrene-butadiene block copolymers and can also be employed to affectthe rubber particle size.

Laboratory demonstrations of rubber particle formation by means of thisinvention can be made using heated microscopic slides. The startingmixture is placed on a slide and the solvent is rapidly evaporated. Forindustrial applications, the starting mixture can be heated underpressure and then rapidly evaporated by exposure to a zone of reducedpressure.

To achieve rubber particles dispersed in a polymeric matrix, the volumeof the rubber phase after evaporation must consist of less than half thetotal volume. Typically, it must be less than twenty-five percent (25%)of the total volume. The weight fraction of rubber in the final productcan be no more than the weight fraction of rubber in the startingmixture expressed on a solvent-free basis. The weight fraction of rubbercan be made less than this by admixing matrix polymer or by polymerizingany unevaporated monomers.

The matrix polymer may also be polystyrene by itself, in combinationwith the rubber which may be polybutadiene. The rubber might also besaturated rubber of the EPDM type which can be used in combination withpolystyrene or polystyrene/acrylonitrile. The matrix polymer might alsobe polyphenyleneoxide or a blend of polyphenylenoxide and polystyrene.The solvent might be polymerizable monomer or monomer mixture.

Surface-active agents for improving the dispersion quality might also beadded into the original mixture. These agents might be AB, ABA, or BABblock copolymers where the A-block is compatible with the matrix phaseand B-block is compatible with the dispersed phase.

A composition of the invention can be formed as a matrix phase composedof a styrene-acrylonitrile copolymer and a dispersed phase comprised ofa lightly crosslinked rubber in the absence of any substantialconcentrations of surfactants other than the block copolymers mentionedabove. In the method of the invention, different evaporation rates canalso be used for different portions of the original mixture in order tocreate bimodal or controlled particle size distribution for thedispersed phase.

A specific example of the invention follows:

Example:

A mixture consisting of

97% ortho-xylene (a solvent)

2.7% polystyrene

0.3% polybutadiene

was formed at room temperature and was allowed to equilibrate forseveral days. The resulting mixture was singe phase.

After equilibration, this mixture was heated to about 245° C. by flowingit through a heat exchanger. A valve at the end of the heat exchangermaintained backpressure and prevented premature evaporation of thesolvent. After the valve, the mixture entered a flash chamber maintainedat about 100 Torr pressure. The heated mixture flash evaporated to givea two phase mixture with the following approximate composition:

50% ortho-xylene

45% polystyrene

5% polybutadiene.

The temperature after flash evaporation was approximately 165° C. Thetwo-phase mixture was gradually cooled to room temperature. Thepolybutadiene was observed to form a dispersed phase having a numberaverage particle size less than 1 micron.

The following requirements must be satisfied to form apolymer-in-polymer microdispersion devolatilization of a compatibilizingsolvent:

1. The pure polymers must be incompatible so that a physical mixture ofthem would form two phases. Polymer/polymer compatibility has beenstudied at length. Most pairs are incompatible. The inventive methodwill not work for a compatible pair. However, it can work for a threepolymer system where a compatible alloy of two polymers is incompatiblewith a third polymer. Two important examples of alloys are polypheneleneoxide with polystyrene and polysulfone with poly(styrene-acrylonitrile)copolymer. A third polymer can be dispersed into such an alloy if allother requirements are satisfied.

2. A compatibilizing solvent or solvent mixture must be found whichintroduces a single phase region into the ternary system of polymerA/polymer B/solvent. Practical considerations suggest that about 1%total polymer soluability is required. This lower limit, however, is notfirm. It is dictated by solvent recovery costs and could be violated ifthe resulting product is especially valuable. It is not possible to finda suitable solvent for all pairs of incompatible polymers but it ispossible for a great many pairs. Solvent selection for screeningpurposes is a straight-forward application of known principles.

3. The compatibilizing solvent must be sufficiently volatile so that itcan be evaporated at temperatures which are not destructive to thepolymers. The ideal solvent would have a low boiling point and a lowheat of vaporization.

4. The volume fraction of the dispersed phase polymer must not be higherthan about 0.2. At higher volume fractions (0.2 to 0.5), large particlesare formed and there is a tendency for both phases to be continuous,that is, the phases tend to be co-continuous.

5. The molecular weight of the continuous phase polymer must bereasonably high so that particle growth by diffusion or agglorimation isinsignificant after the flash evaporation step. In essence, thedispersion is stabilized by the high viscosity of the continuous phase.

These restrictions are quite mild. They can be satisfied by many pairsof commercial polymers and by a host of specialty polymers. If bothpolymers are readily soluable in common organic solvents, it is quitelikely that the method can be made to work. It will obviously not workfor insoluable polymers and may or may not work when one polymer isdifficultly soluable.

These statements and the likely ease or difficulty of getting the methodto work will be obvious to those knowledgeable in polymer physicalchemistry.

The following example systems illustrate the above points:

    ______________________________________                                        Systems Continuous Phase                                                                           Dispersed Phase                                                                            Solvent                                     ______________________________________                                        1       Polystyrene  Polybutadiene                                                                              O--xylene                                   2       Polystyrene  Polybutadiene                                                                              Styrene                                     3       Polystyrene  Polybutadiene                                                                              Toluene                                     4       Polystyrene  Polybutadiene                                                                              Tetrahy-                                                                      drofuran                                    5       Polybutadiene                                                                              Polystyrene  O--xylene                                   6       Polybutadiene                                                                              EPDM Rubber  Tetrahy-                                                                      drofuran                                    7       EPDM Rubber  Polybutadiene                                                                              Tetrahy-                                                                      drofuran                                    8       Styrene-Acrylo-                                                                            EPDM Rubber  1,2 Di-                                             nitrile Copolymer         cholobenzene                                9       High MW      Polybutadiene                                                                              Tetrahy-                                            Alkylated                 drofuran                                            Phenolic Resin                                                        10      High MW      Polybutadiene                                                                              Tetrahy-                                            Coating Resin             drofuan                                     ______________________________________                                    

In all these systems, we have obtained dispersed phase particle sizes of10μ or less when the above criteria have been met. Systems 1-4 showsthat a variety of solvents can be used when they all satisfy thecriteria. System 5 shows that the choice of which polymer is continuousand which is dispersed can be changed at will. Systems 6 and 7illustrate the dispersion of a pair of rubbers in each other. Thesepolymers have glass transition temperatures well below room temperatureand show that the dispersion is stable even though the continuous phaseis technically a liquid. System 8 illustrates a case where finding acompatibilizing solvent was more difficult in the sense that none of theprevious solvent examples would work. However, identification of 1,2dicholobenzene was a straightforward application of known principles.Systems 9 and 10 represents the interesting situation where the methoddid not work when the continuous phase polymer had too low a viscosity.As received, both of the resins were low molecular weight oligimersrather than high polymers. However, they were both heat reactive so thatthe viscosity could be increased through additional polymerization.Following this polymerization, the method worked.

The above list of systems is by no means inclusive. The followingadditional polymers also work with the inventive method: LDPE, PMMA,polysulfone, polycarbonate and polyphenylene oxide. Whether or not themethod works for a given pair of polymers depends on finding a suitablesolvent. It does not depend on any special characteristics of thepolymer beyond having a high molecular weight and being soluable.

FIG. 3 shows the case where polybutadiene is to be combined in a matrixwith polystyrene using O-xylene as a solvent. Curves for evaporation at60° C. and at 140° C. are shown.

In the following table, nine additional combinations are shown whichhave been found to operate in accordance with the present invention.With all of these combinations, microdispersions have been achieved.

These include three additional polymers, namely polycarbonate,polysulfone, and poly(methyl methacrylate). This further evidences thelarge number of examples which can work under the principles of thepresent invention.

    ______________________________________                                        ADDITIONAL MICRODISPERSION EXAMPLES                                           MATRIX POLYMER                                                                              DISPERSED POLYMER                                                                              SOLVENT                                        ______________________________________                                        Polystyrene   Polycarbonate    Chloroform                                     Poly(methylacrylate)                                                                        Polystyrene      Tetrahy-                                                                      drofuran                                       Polystyrene   Poly(methyl methacrylate)                                                                      Tetrahy-                                                                      drofuran                                       Poly(methyl   Polybutadiene    Tetrahy-                                       methacrylate)                  drofuran                                       Polystyrene   Phenolic         O--xylene                                      Phenolic      Polystyrene      O--xylene                                      Polybutadiene Phenolic         O--xylene                                      Polysulfone   Phenolic         1,2 Dichlo-                                                                   robenzene                                      Phenolic      Polysulfone      1,2 Dichlo-                                                                   robenzene                                      ______________________________________                                    

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention, may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method of manufacturing a two phaseco-continuous dispersion of a first polymer in a matrix of a secondpolymer where the polymers are incompatible initially and have highmolecular weights, comprising forming a single-phase mixture of thefirst and second polymers in a sufficient amount of a solvent which iscommon to the first and the second polymers and which does notconstitute more than about 99% by weight of said single-phase mixture,the solvent being selected to be sufficiently volatile so that it can beevaporated at temperatures which are not destructive to the polymers andthe solvent-free volume fraction of the first polymer being from about0.2 to about 0.5, heating the single phase mixture under pressure, andthereafter introducing the mixture into a zone of reduced pressure forflash evaporating the solvent to form a two phase mixture and thematrix, the pressure being selected so that after the flash evaporatingthe two phase mixture contains about 50% solvent.
 2. A method accordingto claim 1, wherein the first polymer is selected from the groupconsisting of polybutadiene, polystyrene, and EPDM rubber.
 3. A methodaccording to claim 1, wherein the second polymer is chosen from thegroup consisting of polystyrene, polybutadiene, EPDM rubber,styrene-acrylonitrile copolymer, high MW alkylated phenolic resin, andhigh MW coating resin.
 4. A method according to claim 1, wherein thesolvent is selected from the group consisting of O-xylene, styrene,toluene, tetrahydrofuran, and 1,2 dicholobenzene.
 5. A method accordingto claim 1, including adding a surface-active agent to the single-phasemixture.
 6. A method according to claim 5, wherein the surface-activeagent is chosen from the group consisting of AB, ABA, BAB block polymerswhere the A-block is compatible with the matrix and the B-block iscompatible with the dispersed polymer.
 7. A composition of matter havinga finely dispersed first polymer in a mixture of a second polymer andmade according to the method of claim 1, the composition comprising adispersion of a first finely divided polymer in a matrix of a secondpolymer which is incompatible with said first polymer, particles of saidfirst polymer being less than one micron in a diameter.
 8. A compositionaccording to claim 7, comprising a matrix phase ofpolystyrene-acrylonitrile copolymer and a dispersed phase of lightlycrosslinked rubber in the absence of substantial concentrations ofsurfactant other than surface-active agents comprising AB, ABA or BABblock copolymers where A-block is compatible with the matrix phase andB-block is compatible with the dispersed phase.